Electrolytic methods for enhancing contrast between metallic surfaces

Abstract

A method of producing a high color contrast between nickel and/or copper or their various alloys and a noble metal or noble metal alloy such as in electrical contacts having a nickel or copper base with an overlay of a noble metal or a noble metal alloy bonded to the surface of the base. By making the contact one electrode in an aqueous solution of a readily soluable ionizable solute and impressing a low AC voltage across the solution, a reaction occurs which makes the base of the electrical contact easily visually distinguishable from the metal overlay.

What is claimed is: 
     
       1. A method of enhancing the visible contrast in electrical contact material of parts thereof which respectively are, first, a substrate consisting of nickel, copper or an alloy primarily constituted by weight of nickel or copper and, second, an overlay bonded to and covering part of said substrate and provided by a noble metallic material selected from the group consisting of the noble metals gold, ruthenium, rhodium, platinum, iridium, osmium and alloys primarily constituted by weight of one or more of such metals, said method comprising, providing an aqueous solution of a readily soluable ionizable solute, applying an alternating voltage of a value in the range from about 3 r.m.s. volts a.c. to about 10 r.m.s. volts a.c. to a circuit comprising said electrolyte, a first electrode constituted of said substrate and overlay, and a second electrode in contact with said electrolyte and spaced from said first electrode, and exposing said substrate and overlay to current produced in said electrolyte by application of said voltage for a time period sufficient to produce noticeable visible darkening of the surface of said substrate relative to the surface of said overlay. 
     
     
       2. A method according to claim 1 in which said ionazable solute is a strongly reactive inorganic acid or a readily soluable, alkali metal or alkaline earth metal salt thereof; a mixture of two or more of such acids; a non-precipitating mixture of two or more of such salts; or a non-precipitating mixture of one or more of such acids and one or more of such salts. 
     
     
       3. A method according to claim 2 in which, when said solute is constituted of an alkali metal salt or alkaline earth metal salt of such an acid or a non-precipitating mixture of two or more of such salts, such solute is present in said electrolyte in a percentage by weight of such electrolyte in the range from about 10% to about 20%. 
     
     
       4. A method according to claim 1 in which said substrate and overlay are exposed to said current through said electrolyte for a period of at least three seconds. 
     
     
       5. A method according to claim 1 in which said darkening of said surface of said substrate is produced by the formation on such surface of a coating of dark particles adhering firmly to such surface. 
     
     
       6. A method according to claim 1 in which said electric contact material is in the form of a length of tape in which said substrate and said overlay form a base and a strip, respectively, for said length of tape, said base being of isoceles trapezoidal cross-section normal to its length, and said strip having a central portion overlying the top of said base portion and, also, border portions which are on opposite sides of said central portions, and which extend down therefrom to cover part of each of the opposite sides of said base, said strip having a thickness which is a small fraction of the height of said base between its top and its bottom.